This invention relates generally to electronic signal processing circuitry and more specifically is directed to an inverting/noninverting differential amplifier circuit in which the differential gain is highly linearized by means of a feedback correction signal arrangement.
In general, a differential amplifier provides two outputs proportional to the difference between the voltages applies to its two inputs. A balanced drive of two inputs may be provided to the differential amplifier or the differential amplifier may be single endedly driven. In the latter case a single input signal is provided to the differential amplifier and compared with a reference signal level in generating the output signals proportional to the difference between the input and reference signal levels.
A differential amplifier generally includes a differentially connected pair of active semiconductor devices, typically transistors which are preferably in integrated form. Since the integrated transistors are highly nonlinear and not well matched in all of their electrical characteristics, common configurations of the differential amplifier still exhibit nonlinearities in gain and matching between their two balanced outputs.
The most common source of nonlinearity is due to transistor emitter resistance dependence upon signal current. Matching between the two outputs is also degraded by output impedance differences relating to circuit gain and .beta..
Additional factors that limit amplitude and phase response matching between the inverted and noninverted outputs of the common differential amplifier arise from signal voltage dependent parasitic capacitances and resistances. In the case of a single endedly driven differential amplifier these parasitic effects are not balanced out or compensated for.
The aforementioned nonlinearities and unmatched conditions in the amplifier give rise to signal processing degradation generally termed distortion. Unwanted signal distortion is sometimes further described in terms of nonlinearities in system operating parameters. The goal, of course, is to minimize distortion from the inputs to the outputs of the differential amplifier in linearizing its gain over a large range of signal frequencies and amplitudes. In the case of a single input, it is highly desirable to provide equal phase delays on its respective balanced outputs.
In prior art differential amplifiers the output signals are generally derived from the collectors of the output transistors. Emitter impedance varies with differential amplifier signal level causing nonlinearities in the amplifier output. As signal amplification is increased, amplifier nonlinearities increase and signal distortion becomes greater. While the prior art discloses differential amplifiers with negative feedback in which the output signals are derived from the collectors of output transistors, these amplifiers are generally limited to a single amplifier stage and have limited frequency response. These amplifiers when single endedly driven suffer from unequal amplitude and phase response resulting from parasitic effects on the driven portion of the amplifier. One example of this type of differential amplifier is disclosed in a document entitled "201:Analog IC Designs" published by Interdesign, Inc., of Sunnyvale, Calif. The aforementioned parasitics are signal dependent and therefore nonlinear and unacceptable in precision signal processing applications.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a feedback gain stabilized differential amplifier possessing highly linear operating characteristics over a large range of signal amplitudes and frequencies. The differential amplifier of the present invention provides compensated inverting/noninverting outputs of equal frequency response and equal phase delay.